Copper tube has always been acceptable for sprinkler systems. The reason it is not commonly used is probably the higher cost of copper installation. This is a partial extract from BS 9251:2021 Paragraph Commentary 5.13 Pipes and fittings: Copper tube conforming to BS EN 1057:2006+A1, R250 should be used above ground. Capillary fittings should be joined by soldering or brazing with alloys with a melting point of not less than 230 °C and conform to BS EN ISO 9453.